Yup, or maybe you accidently caused that Z-Axis vibration during editing the anim.....if you edit an anim in MDLVIS, you can choose if you want the bone to move in any direction or into a specific one. The specific one is always a cupple of axis, like ZX. Maybe you selected ZX during the editing of the movement. Depending from which direction you see the model, there is a chance that by this way of editing you see the bone moving only into one direction, but in reality it will also move around in the second axis of the aforementioned cupple of axis